1. Technical Field
The present invention relates to an artificial knee joint to replace a knee joint.
2. Description of the Related Art
Artificial knee joint replacement is performed in cases in which the knee is afflicted by osteoarthritis, rheumatoid arthritis, or bone tumor, or is subjected to trauma and the like. This artificial knee is a combination of a femoral component that is to be attached to the distal end of the femur and has a medial condyle and lateral condyle and a tibial component that is to be attached to the proximal end of the tibia and has a medial articular surface and a lateral articular surface that support the medial condyle and lateral condyle, respectively. However, both of these condyles and articular surfaces must perform the same movements as a biological knee.
More specifically, when the knee is flexed, the femoral component and the tibial component do not dislocate by slipping to the front and back or right and left. The collateral ligaments and the anterior and posterior cruciate ligaments regulate this slipping to the front and back or left and right, but this ligament function is often destroyed or weakened and slipping readily occurs in patients that are suitable candidates for artificial knee replacement. Therefore, as disclosed in Japanese Patent Application Laid-Open (Kokai) Nos. 2003-116892 and 2003-230582, an artificial knee called a posterior-stabilized type is proposed in which a post is stood in approximately the longitudinal center between the articular surfaces of the tibial component and pushed into the intercondylar groove formed between the medial and lateral condyles of the femoral component, and a cam that will come into contact with the post with flexion at a certain angle is disposed at the posterior portion of the intercondylar groove to regulate slipping to the front and back or left and right.
On the other hand, the problem of rotation with the artificial knee cannot be forgotten. When the knee joint is flexed, the femur rotates laterally (the tibia rotates medially; these are called external rotation and internal rotation). This movement is insured in a biological knee by the joint line (abbreviated JL) connecting the lowest points of the medial condyle and lateral condyle in the medial-lateral (left-right) direction being lower on the medial condyle side (this is called inward inclination) than the SEA (flexion-extension center axis; the flexion center axis when standing, becomes horizontal). However, the artificial knee joint of the above-described example makes deep flexion possible by increasing the turning radius during the final stages of flexion and avoids concentration of stress by increasing the thickness of the cam periphery; therefore, particular attention is not given to the problem of rotation.
The inventors proposed inducing rotation more easily by inward inclination of the joint line of an artificial knee in Japanese Patent Application Laid-Open No. H11313845 and produced commensurate results. However, later research revealed that the PS-type artificial knee has other problems. More specifically, the post and the side surfaces of the intercondylar groove interfere with one another due to rotation during flexion. The post is worn by the interference and the powder from this abrasion invades bones and other tissues, causing them to dissolve and break down biologically. Therefore, this interference is avoided by making the post width much narrower than the width of the intercondylar groove. However, as a result, there is a reduction in strength of the post and the post may break. Moreover, the cam makes firm contact with the post at the end of flexion. If the post width is narrow at this time, there will be a reduction in contact surface area and surface pressure will rise to promote abrasion.